Strain-Mediated Second Harmonic Generation Enhancement\nof Bimetallic Metal–Organic Frameworks

Triggered by the demand of developing\npractical metal–organic\nframework (MOF) based nonlinear optical (NLO) devices, there is a\nsurge of research interest in the regulation of NLO properties of\nMOFs. Strain engineering provides a new opportunity to regulate the\noptoelectronic properties of materials and device performance at the\ninterface. Here we introduce the induction of the strain field in\nstructures by constructing bimetallic Zn/Cu-MOFs. The NLO response\nof MOF1–9 with different metal element ratios was characterized\nin the near-infrared window. Surprisingly, MOF3 exhibited a giant\nsecond harmonic generation (SHG) enhancement compared with that of\npristine Zn- and Cu-MOF. The determined second-order nonlinear susceptibility\nof MOF3 (∼19.86 p mV<sup>–1</sup>) is one order of magnitude\nlarger than that of pristine Zn-MOF (1.65 p mV<sup>–1</sup>) and Cu-MOF (3.88 p mV<sup>–1</sup>) and much higher than\nthe commonly used NLO crystals. Moreover, the anisotropy ratios of\nMOF1–9 can be harnessed in a wide range (∼4.7–118.8).\nThe anisotropy ratio of MOF4 (∼118.8) is much higher than that\nof the most excellent 2D material to date, to our best knowledge.\nA first-order perturbation of second-order polarization based on point\ngroup description was introduced to explore the regulation mechanism\nof the NLO response under the strain field of MOFs.